In an optical communication system, optical signals are modulated to carry data and transmitted on an optical communication path, including an optical fiber, for example, from a transmit node to a receive node. At the receive node, the data is detected and provided to a user, for example. In wavelength division multiplexed (WDM) optical communication systems, multiple modulated optical signals, each having a corresponding wavelength, are transmitted from the transmit node to the receive node along the optical fiber.
In point-to-point WDM optical communication systems, all the optical signals output from the transmit node propagate along a single fiber optic link and are terminated at the receive node. However, optical communications including multiple links that are interconnected with one another have been deployed that have greater capacity and flexibility. At nodes interconnecting two or more links, optical signals may be dropped and may be sensed by optical receivers. Additional optical signals, often having the same wavelengths as the dropped optical signals, may be added or transmitted to another node. An optical add/drop multiplexer may be provided at a node interconnecting multiple links in order to facilitate such adding and dropping of optical signals from one link to another.
In certain optical add/drop multiplexers, the optical signals that are added and dropped is fixed. That is, the optical signals are typically not changed once the optical add/drop multiplexer has been deployed. Other add/drop multiplexers, however, are reconfigurable in that the number optical signals that can be added and/or dropped can be changed. Such reconfigurable optical add-drop multiplexers (ROADMs) may be provided where multiple optical fiber links converge, and a user desires to add and drop optical signals depending on capacity requirements of the WDM system and/or other parameters.
ROADMs may include banks of optical transmitters and receivers that provide the added optical signals and the dropped optical signals, respectively. In high capacity systems, individual signals may be grouped into so-called “super-channels” that are routed through the optical communication system as a single unit. The transmitters, outputting such optical signal groups, may be coupled to a switch, which may include individual switches that selectively direct the optical signal groups supplied by the transmitters to particular optical combiners or multiplexers. The optical combiners, in turn, combine those selected optical signal groups, which have been designated to be added to a given optical communication path. The switches may also direct other optical signals to other combiners that direct such other optical signals to another optical communication path, for example.
The added optical signal groups may incur a loss, however, while propagating through the optical combiners, as well as other optical components in the switch. Accordingly, optical amplifiers, such as erbium doped fiber amplifiers, may be provided in the ROADM to boost the power of the optical signals and thus offset the incurred loss.
Erbium doped fiber amplifiers (EDFA) often generate amplified stimulated emission (ASE) light at wavelengths other than the optical signal wavelengths. Such ASE light is generally regarded as noise, which should be minimized in order to accurately detect the optical signals in the receiver. In the optical combiners, in which first and second groups of optical signals are to be added, however, the ASE light generated by a first EDFA amplifying the first group of optical signal may also be present at wavelengths corresponding to the second optical signals. Thus, when such ASE light is combined with the second optical signals in the multi-cast switch, the signal quality (e.g., the optical signal-to-noise ratio, OSNR) of the second group of the optical signals is undesirably decreased. As a result, the number of errors, e.g., the bit error rate, of the second group of optical signals may increase. Likewise, the bit error rate of optical signals in the first group may also increase due to ASE at the first optical signal wavelengths generated by a second EDFA that amplifies the second group of optical signals.
Although the OSNR associated with the combined optical signal groups may be acceptable if the optical signal groups propagate over relatively short distances, the OSNR may increase to unacceptable levels after transmission over longer distances. Accordingly, the reach or transmission distance of such combined optical signal groups is limited.